Also known as purple witchweed, this plant is characterized by bright green stems and leaves as well as small, bright, purple-colored flowers. This plant parasites the crop and extracts water and nutrients from the host plants, resulting in symptoms that mimic those of drought stress or nutrient deficiency: chlorosis, wilting of leaves and stunting of the plant. Pre-emergence symptoms are difficult to diagnose precisely because of their similarity to a general lack of nutrients. Once the emergence of the striga has taken place, it is usually too late to mitigate the damage, even if it is pooled out. This can lead to significant loss of yield.
The symptoms are caused by the parasitic plant Striga hermonthica, commonly known as purple witchweed or giant witchweed. It is particularly problematic in sub-Saharian Africa. They can be a serious problem to crop cereals, grasses and legumes including rice, maize, pearl millet, sorghum, sugarcane, and cowpea. Each plant can produce between 90,000 and 500,000 seeds, which can remain in the soil for periods of over 10 years. These seeds overwinter in the soil after their dispersal by wind, water, animal, or human machinery. When weather conditions are favorable and they are within a few centimeters of a host root, they will begin to germinate. Once in contact with the root, the witchweed produces a structure that attaches it to the host plant, thus establishing a parasitic relationship. Nitrogen-rich fertilizers also reduces the witchweed infection rate.
Witchweed is among the hardest parasitic plants to control, mainly due to the high amount of seeds it produces and their longevity. Affected plants should be uprooted and burned before they flower. The fungus Fusarium oxysporum may be used as a possible biocontrol of witchweed since it infects the early vascular tissues of the Striga plant and impairs its growth.
Always consider an integrated approach with preventive measures together with biological treatments if available. Herbicide against the witchweed are available are rather expensive and they may also directly affect the crop. The sprayer needs protection and the herbicides can kill useful plants. Herbicide priming of seeds has been used successfully in millet and sorghum with up to 80% decrease in infestation. It consists in soaking herbicide-resistant seeds in herbicide solutions before planting.